Compromise camshaft timing has been generally accepted, that is, a positive drive between the crankshaft and the camshaft. However, this has meant considerable sacrifice of performance of internal combustion engines since for low rpm an early intake valve closing is desirable, while at high rpm it is clearly preferable to retard the intake closing point, even to leaving the intake valves open for a short time after the pistons reach the bottom of the intake stroke. This allows the in-rushing air-fuel mixture to literally supercharge the cylinder. At least two proposals have been made for automatically changing the cam timing for high speed operation, namely the 1965 device sold under the mark CAM-A-GO which substitutes an idler gear between camshaft and crankshaft gears and provides for shifting this idler to retard the timing at high speed. The other proposal is disclosed in applicant's earlier patent which discloses a resilient drive coupling between a positively driven sprocket and a plate drivingly connected to the camshaft, said plate being mounted on the sprocket for free rotation, within limits, under the sole discipline of a spring which biases the plate into a position representing an advanced timing position of the camshaft.